The invention relates to a pen comprising    a cartridge,    a retaining sleeve for at least partially accommodating the cartridge,    a locking device which can be actuated by rotation for locking the cartridge and the retaining sleeve together, and    a cap for protecting the cartridge, wherein    the cap is axially displaceable with respect to the locking device from a first operating position in which it is rotatable with respect to the locking device into a second operating position in which it is rotationally coupled to the locking device.
Pens of the kind set forth in the opening part of this specification are known, for example from DE 36 44 978 C2. In that arrangement the cap has a sealing insert which sealingly encloses a writing tip of the cartridge. The cartridge is held stationarily with respect to the locking device. When the cap is moved into the second operating position the sealing insert is displaced along the writing tip.
The above-outlined structure has been found to be unsatisfactory, in particular in regard to lack of sealing integrity.
DE 36 40 518 C1 discloses a comparable pen. In that case however the cap is not displaced approximately axially but is compressed radially for moving it from its first operating position into its second operating position. That way of handling it is found to be impractical. In addition there is the disadvantage with this structure that the cap must be of a comparatively large diameter so that it can be radially compressed.
The object of the present invention is to overcome the above disadvantages and in particular to provide a pen in which the cap reliably seals off the writing tip of the cartridge but which is nonetheless simple to handle and which takes up as little space as possible.